This invention relates to bone fixation plates, and particularly to internal or implantable fixation plates suited for stabilization of pelvic bones.
Internal or implantable bone plates are well known for stabilization of bone fractures. Most bone plates are elongated plates which include at least one flat and usually straight surface forming a bearing surface for bearing against the bone fragments to be stabilized. Some plates are curved or concave to conform to the curvature of the bone structure, and some bone plates have more than one elongated section, such as plates having "Y" shapes. Most bone plates include apertures for receiving pins or bone screws whose heads bear against the plate to affix the plate to the bone fragments to be stabilized. The plates also come in a wide variety of lengths and have a varying number of bone screw apertures. Examples of such bone plates may be found in U.S. Pat. No 2,441,765 to Hopkins, U.S. Pat. No. 3,593,709 to Halloran, U.S. Pat. No. 4,219,015 to Steinemann and U.S. Pat. No. 4,573,458 to Lower.
Pelvic bones tend to vary in size and shade between patients, thereby making it difficult to produce a single bone plate universal to all patients. Consequently, it is necessary to adapt bone plates to the specific requirements of patients by bending or twisting the plate to fit the contour or configuration of the patient's bone. Therefore, one important feature of most bone plates is that the plate must be capable of being bent to conform to the specific contour of the bone to which is to be attached. Most bone plates are designed to permit up to 15.degree. bending or twisting. However, bone plates are most likely to break or fail at the region of the bone screw aperture where the cross-section of the material is reduced and the plate has lower tensile strength. Stress across a screw aperture on a flat plate tends to be increased by a concentration factor of nearly two. Consequently, care must be taken to avoid twisting or bending a plate at a screw aperture.
Another factor in bone plates is that the plate should have a bending resistance matched to the bone to be stabilized. If the plate is too flexible, the region of the fracture is not sufficiently stabilized, resulting in inadequate restoration. conversely, if the bone plate is too stiff, the plate takes up most of the loading forces which leads to general decay of the bone. However, the bending resistance of a bone plate is altered by forces (including friction) at the interface between the bearing surface of the bone plate and the bone surface, as well as by forces transmitted between them.
One pelvic bone plate, which is commercially available from Zimmer, Inc. as product number 1179-00, employs an elongated elliptical rod having support tabs spaced laterally along the length, the tabs having bone screw apertures for mounting the plate to the bone. This product, which is shown and described in conjunction with FIG. 6 of the aforementioned Lower patent, employs tabs which are significantly thinner than the thickness of the elongated rod portion. According to the Lower patent, the elliptical design reduces stress in the bone plate caused by twisting or bending the plate. Moreover according to Lower, the placement of the screw apertures on the lateral tabs (off the plate axis) permits distribution of stress along the length of the plate as the plate is twisted or bent.
One problem with the Lower/Zimmer design is that the laterally offset screw fasteners fastening the plate to the bone creates a bending moment in the tab which tends to concentrate stress at the intersection of the tabs and rod. Since the tabs are relatively thin, the risk exists that the tabs may sever at the junction between the tabs and the rod. Moreover, the underside of the rod portion is relatively flat and coplanar with the lower surfaces of the tab portions to form a continuous bone bearing surface along the length of the rod which encompasses the tabs. The flat bearing surface alters the intended distribution of stress caused by bending and twisting, thereby causing uneven stress distribution. Hence, stress is actually concentrated at the intersection of the tabs and rod, thereby leading to failure of the plate.